Backlash eliminator



Feb. 16, 1937. E. G. ROEHM ET AL 2,070,807

BACKLASH ELIMINATOR Filed Oct. 20, 1 933 4 Shets-Sheet 1 awe/Mow EKW/N 6 FOE/1M Wm TER I. REC/0519 Feb. 16, 1937. E. ca. ROEHM ET AL 2,070,807

BACKLASH ELIMINATOR Filed Oct. 20, 1935 4 She'etsSheet 2 fRW/N 6. ROEHM WHLTER l7. HRcHEfl 193?, E. 5. ROEHM ET AL 2,07,,6?

BACKLASH ELIMINATOR Filed Oct. 20, 1933 4 Sheets-Sheet S m3 WHLTEE 17. EEC/{E19 nae Erwin G.

tion ol Ohio Application flctoberw,

operative and effective during thefeeding movement of the table and which is ineflective during quick traverse movement.

Another object of the invention is to provide I an eificient back lash eliminator for a moving support, wherein the back lash eliminator is equally effective when said support is moved at l5 feed rate in either direction, and wherein the production of back lash elimination tendency is higher when the feed resistance is light, and the tendency to vibration and chattering of the support is great, and conversely, wherein the resistance efiect of the back lash eliminator automatically decreases under conditions of higher feed resistance when vibration andchatter of the support is reduced because of the very nature of the cut.

in a machine tool, hydraulically controlled means for manually and automatically effecting desired cycles of support operation including feed and quick traverse movements in either-direction and stops, and cooperating back lash eliminating mechanism hydraulically and automatically rendered operative during feed rate movement only of the support or table.

An additional object of the present invention is the provision of a mechanism which will impose. a restraint on movement of a machine. tool support when being driven at a feed rate, and/or when the feeding impulsion'is disconnected therefrom whereby maximum accuracy of stopping or reversing of the support is insured and tendency to over-run due either to inertia or cutter action counteracted. l

These and other objects are attained by the means described herein and disclosed in the accompanying drawings in which: a

Figure 1 is a perspective view of a milling machine having the mechanism or the invention embodied therein. Figure 2 is a schematic hydraulic circuit diagram embodying the several automatic controls for the machine shown in Figure 1. Figure 3 isa plan view showing the back lash eliminator mechanism orthe invention as a plied to a rotating 'nut feedfor a sliding work supportin: table;

A further object of the invention is to provide 1m and Walter ill). Archw, Ohio, assignors to Elie Gincati chine Company, Cine 1933, Serial st. states (on. so -22) Figure 4 is a plan view of a rack and pinion type of feed drive embodying the back lash eliminator of the invention.

Figure 5 is a plan view showing a worm and worm gear type of drive embodyingthe means 5 of the invention.

Figure 6 is a schematic view showing the power source and mechanical controls governing the di-'- rection of feed-of the work supporting table, the table feed clutch, and rapid traverse drive for 10 the machine shown inl 'igure i.

In milling machines wherein the material acted upon is carriediby a movable table or work sup,- port to a rotary cutter, it is desirable for production purposes'to control both table movement 15 and cutter operation by automatic as well as by manual control means. Furthermore, in cutting operations'where the direction of effective movement of the engaging teeth of the cutter and the direction of relative feed of the work and cutter is the sam and also in race milling operations wherein the surface of the workoperated upon is unequally distributed in parts relative to the cutter, it is necessary to overcome the back lash or tendency to lost motion between the sup- 2;; port and the actuating means therefor. This -so-called back lash causes chattering and vibration of the support and results in inaccurate and/or tooth-marked work if not properly eliminated, and jumping of Ethesuppcrt as the 30 hook-in cut becomes effective.

The milling machine 'of the presentinventlon is adapted to accurate and speedy production operations of all kinds and to this end embodies a combined automatic hydraulic and selective man- 35 ual control for the feed and quick traverse movements of the tablein either direction and automatically and hydraulically controlled back lash eliminator mechanism efiective to the necessary and proper degree, according to the reed resist- 40 ance to the movement of theta le.

Figure 1 showsa milling machine equipped with hydraulically operatedautomatic controls and embodying the back lash, eliminator of the invention. I .45

The machine consists essentially of a base l2, column l3, having an overhan ing arm ll which supports the free end of the cutter arbor l5. A

feed screw 16 cooperating with power. driven nuts and I8, (Figure3) serves toeiiect longitudinal movement in either direction of a sliding work supporting table 18. 1A bracket housing 20 at the front of the machine houses the hydraulic valve, mechanism and the combined manual and automatic control means therefor. Control lever 55 2| projects from housing 28 and is the means whereby the manual control of the machine is effected. An auxiliary dog rail 22 is 'adjustably and removably attached to the work support table l 9 and carries adjustable dogs for cooperation with trip plungers extending from the rear of housing 28.

Referring now to Figure 3 a pair of bearing bosses 23 and 28 integral with bed l2 are provided. with thrust bearings 25 against which the feed nuts I! and I8 abut and through which the cooperating feed screw I8 extends.

The back lash eliminator mechanism is interposed in the table feed transmission just ahead of the table feed device as will later be fully explained.

In order to simplify the explanation of the ma- I chine embodying the invention the feed and quick traverse transmissions for the table will be described first in connection with the schematic view in Figure 6.

As shown in Figure 6, the power from a suitable source, such as motor 28, serves to operate the main feed drive shaft 21 through'rotatable members 28 and drive band 29. The main drive shaft has a feed clutch 38 controlling the transmission of power through suitable gearing 8i and pick-off gears 88 and 82 to'a final drive shaft 32. The shaft 32 drives shaft 33 through a suitable reversing clutch 34 which may be alternately engaged to transmit clockwise or counter-clockwise rotation to shaft 33. An overrunning clutch on shaft 32 and a gear 36 on the same shaft and cooperating with a clutch controlled rapid traverse drive gear 31 on shaft 21 together with a feed brake 13! complete the mechanical transmission controls of the-machine. A rapid traverse clutch 4| serves to connect gear 31 to shaft In the above mentioned drawings, there are illustrated several characteristic embodiments of the back lash eliminator of the invention, and also the adaptation of one of them to the automatic type of milling machine, the latter adaptation being illustrative of the utility of the back lash eliminator in hydraulically controlled machines and of certain novel control means as well.

Referring to Figure 3, feed nut IT has an integral or attached right hand helical gear with the teeth thereof inclined at an angle of the order of forty-four degrees (44). The nut l8 has a left hand helical gear 84 integral therewith or fixed thereto and having the teeth thereof inclined at an angle of the order of thirty-two degrees (32). Gear 89 has the same ratio in number of teeth to gear 84 as gear 88 has to gear 85. Gears. 84 and 85 have an equal number of teeth. A spool 86 is splined on hollow shaft 81 and has fixed gears 88 and 89 on its opposite ends matching the gears 85 and 84 respectively. The spool has a longitudinal slot 98 through which a pin 9| mounted on plunger 92 may operate under the influence of hydraulically controlled piston 93 to exert pressure axially of the spool. Gear 94 is suitably keyed to shaft 81 and is driven by a gear 95 on shaft 98 which receives its power at suitable speed and direction of rotation through reverser shaft 33 of which shaft 98 is a continuation.

When the rapid traverse mechanism is operatlve as in shifting the work supporting table 76 the spool is thus free to float endwise, rotation of erted on piston 93 in chamber 91.

gear 88 will be resisted by whatever load is on gear 85 and due to the spiral angle of the teeth, there will be a reaction tending to shift gear 88 to the left. This movement will cause relative axial movement of gear 89, which due to its spiral being reversed to gear 88, will tend to cause opposite rotation of gear 88 which will form a reaction to further movement and balance the drive through the two branches.

In climb cutting operations, and in face milling operations wherein the surface of the work operated on is unequally distributed in parts relative to the cutter, the back lash or tendency to lost motion between feed screw l8 and the drive nuts I! and i8 is overcome by exerting an axially or longitudinally directed pressure on the spool 88 through the agency of hydraulic pressure ex- 89 having'the teeth thereof at different degrees of angularity and of opposite direction of angularity have unequal amounts of end thrust in opposed directions in their cooperative relation with their respective companion gears 85 and 84, respectively. By exerting the proper amount of pressure axially of the spool 86,- and in a direction opposing the greater directional end thrust of gear 88, the slight amount of clearance between the threads of the feed screw and the threads of the drive nuts l1 and I8 is taken up so that one nut bears against one side face of the threads and the other 'nut against the opposite side face of the threads of the screw, and no back lash or lost motion exists. There is thus eliminated any tendency to chattering of the machine or undesired movement of the controlled support. When the direction of rotation of spool 86 and its gears is clockwise (viewing the gears from theright hand end of Figure 3),the greater and thrust of gear 88 is to the left hand side and the lesser end thrust of gear 89 is toward the right hand side. Under such conditions the hydraulic pressure is imposed on the left hand face of piston 93 to oppose the end thrust due to the steeper angle of teeth on gear 88. If this pressure in cylinder 91 is just sufficient to overcome the tendencyto and movement of gear 88, or in other words sufficient to hold gear 88 into tight driving contact with 85, the nut ll will be held advanced due to the angularity of the intermeshin-g spiral teeth as far as possible, so that the nut I! will be in tight engagement with one angular side of the screw thread and so that any further rotation must cause axial movement of the screw. Due to this condition, the gear 89 being integral with gear 88, is also axially moved, but the angularity of its teeth being reversed to that of gear 88, will cause reverse rotary movement of gear 84 to its limit of movement so that the nut I8 is in tight engagement with the opposite side of the screw thread. It will now be seen that no back lash exists, and an external force acting to Gears 88 and cause movement of the table will be resisted by nut l8; but at the same time the nut I1 is maintained in a potential positive driving condition so that at any time the external force momentarily diminishes, 'or disappears entirely, the table .will still be moved at the same rate without interruption. As a matter o fact only a certain range of hook-in cuts, such as up to a certain depth for instance, cause this acceleration tendency of the table over the desired feed rate. But this acceleration tendency of the cutter is not steady and uniform but intermittent in nature, ranging from low pressures incapable of causing acceleration of the table to momentarily extremely high pressures. Therefore, by having one member in a driving position and the other member in a holding back position, a uniform feed is obtained. Should the depth of cut increase beyond this range due to unevenness of the work piece, the acceleration tendency of the cutter will not only reduce to zero but a great re sistance to table movement might be created so that a load is thrown on the table drive. Should this load become so great, that the axial component of gear 88 is greater than the opposing hydraulic pressure, then the sleeve 86 will shift against the resistance of the hydraulic pressure causing gear 89 to rotatably accelerate gear 80 from a holding back or retarding position into an intermediate or neutral position, and finally into a driving position'causing nut I8 to assist the nut il in feeding the table. Since the hydraulic pressure is still active, it will be seen that simultaneously with a decrease of table load, the sleeve will shift again to move nut I8, through the medium of gears 84 and 89, to a back lash eliminating position. This important feature of the invention not only makes it possible for nut I8 to act as retarding means to eliminate back lash but also to act automatically whenever the necessity arises to assist in the actual driving of the table. When the direction of rotation of the spool 86 is reversed, for the purpose of feeding the table I 9 in the opposite direction, the greater end thrust of gear 88 is toward the right and hydraulic pressure is applied to the right hand side of piston 93 to oppose this greater pressure to prevent locking of the parts. When rapid traverse drive of the work supporting table takes place, no hydraulic pressure is exerted on the piston and the spool gear and the reaction of the drive on the gears in the absence of cutting load tends to centralize 80 and cause relatively free movement of the support.

In the embodiment shown in Figure 3, which is the form used in the machine of Figure 1, the gears and 00 are co-axial. In the modifications of Figures 4 and 5, it will be noted that the corresponding gears need not be co-axial, but that various arrangements may be had to accommodate the back lash eliminator to different types of mechanism.

The same general arrangement can be used with a rack and pinion drive, for example, as shown in Figure 4. The back lash eliminator in this embodiment may constitute spool gear 98 splined on shaft 99 which is at right angles to a rack I00. Shaft 99 is driven from shaft IOI through the agency of miter gears II02 and I03.

The spool gear is subject to longitudinal shifting by hydraulic pressure admitted to chamber I 00 through suitably controlled pressure tubes I05 and I00 at opposite sides of piston MI. The piston rod I08 may have connection with the slidable spool 90 similar to that illustrated in Figure 3. The spool 90 has helical gears II09 and l I0 at the opposite ends thereof, the gear I09 having its teeth at greater angularity than and in opposite direction to the teeth of gear H0. Gear I09 is a right hand gear with the teeth preferably disposed at an angle of about forty-four deas working conditions render elimination of back-' lash desirable, as when cutting-,hydraulic pressure is admitted through pressure tube I05 or I06 to oppose the greater end thrust of gear I09 sufiiciently to restrict or limit the relative movement of the gears to an amount just sufficient to accomplish this adjustment of the pinions with respect to the rack teeth.

In Figure 5 there is shown the adaptation of the back lash eliminator of-the invention to a worm and worm gear drive. In this embodiment the hydraulically controlled spool III has the gears H8 and II9 thereof arranged with the teeth in opposite and unequal angularity, for example as explained in connection with Figures 3 and 4. Gear II8 meshes with gear I on shaft I2I which carries worm I22 meshing with worm wheel I23. a shaft I25 which may be parallel but not necessarily aligned with shaft I2I. A pair of miter gears I26 and IN on the shafts I25 and I28, respectively, serve to effect rotation of worm I29 meshing with a second worm gear, located behind worm I23 and therefore not shown. Through the proper adjustment of hydraulic pressure through the device indicated generally at I30, the clearance between the threads of the respective worms I22 and I29, and their intermeshing worm gears may be adjusted according to the direction of drive in a manner which will be well understood from the description of the backlash eliminator in connection with Figures 3 and 4.

Referring now to the machine shown in Figure 1 which employs the back lash eliminator mechanism illustrated in Figure 3, it will be explained in connection with Figure 2, how this eliminator may be interconnected and operated by the automatic hydraulic and selective manual control so as to function in accordance with the different cycles. I 7

It will be understood that the feed clutch 30, feed brake I 3I, rapid traverse clutch 4|, directional reverse clutch 30 and back lash eliminator control piston 93 are all hydraulically actuated through suitable pressure lines receiving oil from an oil sump or reservoir I32 through pump. 40 and a main hydraulic pressure line I33. Valves controlling auxiliary pressure lines to the several instrumentalities are controlled manually through the lever 2I; and associated trip plunger mechanisms are interconnected therewith and subject to automatic actuation by plungers operable by a group of control dogs mounted on the adjustable dog rail 22.

The pressure supply line I33 has a connection I30 to a direction valve mechanism I35 which is adapted for two positions wherein pressure is passed selectively into line I36 or I31 which control a hydraulically actuated piston I38 for the directional clutch 30 which is on shaft 33. Thus one end or the other of the piston I38 is always subjected to the supply pressure, while the opposite end of the piston is connected at valve I35 with one of the return lines of the sump indicated through a triple auxiliary valve MI with a second control valve I42. Valve I02 which may be termed a feed and quick traverse selector valve, has three positions corresponding respec- Gear II9 meshes with gear I24 on tively to rapid traverse, feed, and stop. The rapid traverse connection is. through line I43, valve I44, and line I45 to the rapid traverse control 31. In the position shown the rapid traverse control is operative through line I43; while line I45 to feed brake I3I, and also line I46 to feed clutch brake 300 are without pressure. Feed clutch 35 is of the spring opened and hydraulically closed type. In the position shown, the valve piston 3I1I is under the influence of its spring by reason of no pressure in line I46 which communicates with the opposite end of the piston. A branch line 332 of line I33 at this time directs its pressure through line 303 to urge feed clutch 33 to engaged position. A by-pass 354 to the sump is closed at this time. Further, the piston 93 of the back lash eliminator in chamber S'I is not under pressure because line I51 is connected through by-pass I I8 to sump connection I49 and line I53 from the opposite end of chamber 91 is connected through by-pass II and line I52 through resistance I53 to sump connection I55. Valve I55 has connections at its opposite ends through lines I55 and I51 with lines I55 and I 31, respectively. Thus valve I55 is always conditioned in accordance with the direction valve I35 and with the direction clutch control piston I38 to direct the pressures from line I52 to the proper end of cylinder 31 to cause the back lash eliminator to act automatically in the correct direction in accordance with the direction of feed. An automatic spindle stop mechanism may also be provided to act in proper coordination with the back lash eliminator whereby the spindle will be automatically stopped when the back lash eliminator is off as when the table is moving at'rapid traverse; and rotating when the back lash eliminator is on as when the table is connected to the feed mechanism. An electric limit switch 335 is provided for starting and stopping the spindle actuating motor. This switch is moved to its stop position, shown in Figure 2, by an hydraulic actuated piston 351 through in tervening lever 355. Fluid pressure is supplied through line 358 for shifting piston 361 to the left, this line being connected through cannelure 359 in valve 3IE| to rapid traverse supply I63. The fluid acting on the other end of piston 3511 is exhausted through bore SI I, adjustable valve 3I2, and line 3I3 which is connected at this time by cannelure 3II in valve 3I5 to line I55 in turn connected to exhaust at this time as previously explained. Pressure is applied to line I52 when valve I42 is shifted upwardly to its feed position to close the rapid traverse supply line I53 and connect the pressure supply branch I 58 of the main line through port I53, line I55, through branch line I65, pressure reducer HM and thence through I52 connected to the pressure reducer I6 I. The amount of pressure drop may be varied by manually adjusted cam 325 acting to vary compression of spring 325 which is opposed by the pressure acting on the upper end of the valve plunger. If valve I55 is positioned, as shown, with the line I52 communicating with valve groove I5 I then pressure is directed through line I511 to the left-hand end of chamber 91, the opposite end at this time exhausting through line I41, by-pass I48, to sump connection H19. If the direction valve I35 is shifted, then the pressure would be in the reverse direction, in chamber 91, entering through line I52, passage I48 and line I41 to the right hand end of chamber 91 and exhaust would pass from the left hand end of the chamber 91 through line I50, by-pass I5I, and sump connection I62. It will now be seen that the backlash eliminator piston is automatically subjected to pressures whenever the feed clutch is engaged and the rapid traverse clutch disengaged. Furthermore, it will be noted that since line I43 is now a return and line I45 a pressure chamber, flow through lines 308 and 3I3 will be reversed, the pressure fluid entering line 3I3 forcing wide open the needle valve to provide free fiow to bore 3H, thereby shifting plunger 351 to the right and actuating switch 305 to start the spindle motor.

When valve I52 is again shifted upwardly to its third or stop position, the continuation I53 of branch main I58 opens up pressure communication through port I55 to line I55, while still maintaining pressure connection to line I45 but connecting line I53 to reservoir.

The pressure in line I55 moves a valve piston 3M against its spring to close line 352 and at the same time connect line 353 with by-pass return 355 to release clutch 35 under the influence of its spring. The feed clutch brake 3I3II operates at this time to avoid overrunning of the table movement. It will thus be seen that although pressure is still on in line I55 which normally would cause the spindle to rotate, pressure in line I 35 reverses the connection in such a manner that the spindle is stopped. Also it should be noted that when valve I62 is shifted from its rapid traverse position to its stop position without a stop at the feed position, that although the backlash eliminator is off or ineffective during rapid traverse, it becomes efiective when the table is stopped. This insures against lost motion should the table be manually adjusted as for setup purposes and that the table will be accurately positioned. Means have been providedwhereby the operator may selectively determine for himself when the backlash eliminator is or is not effective. In the construction thus far described, the backlash eliminator is always efiective during teed regardless of direction. in cases it may only be desired when feeding in one direction such as feeding to the left only, or feeding to the right only; or it may not be desired at all. These various adjustments are effected by a rotatable valve 3 I 5 having a pair of arcuate shaped grooves 3M5 and 3I1l of sufiicient length when the valve is in a central position to connect lines I51 and I55 to branches 3% and 3i 5 communicating with selector valve 323 which has two plungers MI and 322, reciprocably mounted therein for movement against a spring 323. Thus when pressure is on line I31], pressure in line 3I8 will act on the end of plunger 32I to move plunger 322 to efiect inter-connection between lines I55 and I55. Likewise, when pressure is on line I35 due to change in direction of table movement, pressure in branch 3I3 will act on the end of plunger 322 to lift the same and interconnect line I55 with I45. Thus, the back-lash eliminator is efiective in both directions of feed and in the manner previously explained. Clockwise rotation of valve 3I5 a predetermined amount will maintain connectlon between line I31 and branch 3I8, but will disconnect branch an from line BIG and connect it to return line 324, thus making the back-lash eliminator effective only in the direction of feed determined by line I31.

counterclockwise rotation of valve M5 the same amount will reverse the connections so that backlash eliminator is only effective during the direction of feed determined by line I35.

If the valve 3I5 is rotated still further beyond aovaeov the positions described, both branches tit and 3M are disconnected from their respective central lines 3 l t and 3 l l and the back-lash eliminator is not efiective at all.

The valves I35, HM and M2 may be operated by means of the following mechanism:

An oscillatable and longitudinally reciprocable sleeve M9 is provided for actuation by the manually operable lever 2i. This sleeve is connected by means of the double bell crank N33 to plungers i355 and Ni, and in such a manner that when the valve 035 moves in one direction the valve plunger Ml moves in an opposite direction. Rotary oscillation of sleeve I69 actuates through the pivoted lever IN the control valve plunger M2.

Besides being manually operated by the lever M, the sleeve it may be automatically actuated by the axially movable and rotatable trip plunger ltd. An auxiliary stop plunger 11% may be provided in addition to the trip plunger me for automatically shifting the same to a stop position. This plunger may be provided with a pawl ld'i pivotally mounted within a slot formed in the plunger and urged outward by a spring pressed pin ltd; The end of this pawl is thus held in the path of a shoulder 20d of a pivoted arm 285 which carries near its free end a lug 202 which engages in a groove 203 formed in plunger i188. Since the form of the automatic trip mechanism for actuating these valves forms no part of the present invention, further description thereof is not believed to be necessary.

There has thus been provided an improved mechanism for eliminating backlash from a machine tool transmission and which may be selectively rendered operative or inoperative and either automatically or manually in accordance with the cycle of operation of the machine.

What is claimed is:

1. In combination with a hydraulically controlled machinetool having a movable work feed table and hydraulically controlled power means for effecting selected movements of said table at selected speeds in opposite directions, of back- :l lash eliminator mechanism operative on the table moving means at certain speeds and comprising constantly engaging drive means including a pair of driving helical gears of opposite and unequal tooth angularity rotating as a unit, hydraulical means to exert axially directed pressure to shift said unit and a pair of independent helical gears meshing respectively with the gears, of said unit and driven at uniform speed thereby, the longitudinal shifting of the unit effecting a limited at diii'erential rotational adjustment of said independent gears, independent feed means operable respectively by said independent helical gears and a feed element engaging with said pair of independent feed means.

2. In combination with a pair of relatively movable members, of a pair of rotary feed elements on one of said members, a complementary feed element on the other of said members engaging with said pair of feed elementsand back lash eliminating mechanism whereby one'oi said pair of feed elements may be retained in advanced engaging relation to the complementary feed element tooppose back lash between said relatively movable members, and comprising a pair of hellya cal gears of opposite and unequal tooth angularity means to selectively rotate the driving member in opposite directions and means to apply pressure axially of the shiftable driving member to limit the advanced engaging relation of the said feed element.

.3. In combination a pair of relatively movable members, feed means to efiect movement of one of the members relative to the other, and comprising a single feed element on one member and a pair of feed elements on the other member engaging the feed-element on the first mentioned member; and anti-back lash mechanism comprising helical gears of opposite and unequal tooth inclination respectively on said pair of feed elements, a longitudinally shiftable power actuated rotary spool having at'its opposite ends teeth meshing respectively with the teeth of said helical gears and means to exert an axially directed pressure to said spool.

4. The -combination with a-pair of relatively movable members, of a feed mechanism for effecting relative movement between said members and comprising a pair of rotary driven elements carried by one of the members and a member on the other movable member cooperating with said pair of rotary elements, a rotating and longitudinally shiftable. spool, gear teeth at opposite ends of the spool, the teeth at one end being inclined in one direction, the teeth at the opposite end being oppositely inclined and at a difierent angle of inclination from the first mentioned teeth, companion gears meshing with the teeth at opposite ends of the spool, said gears effecting movement of the respective driven rotary elements, and means to exert axially directed pressure on the spool against the thrust of the meshing gears of greater teeth angularity.

5. The combination with a pair of relatively movable members, of a feedmechanism for effecting relative movement between said members and comprising a pair of rotary driven elements carried by one of said members and a member on the other of said members cooperating with said pair of rotary elements, a spool member shiftable longitudinally and rotatable in opposite directions, gear teeth at opposite ends of the spool, the teeth at one end being inclined in one direction, the teeth at the opposite end being oppositely inclined and disposed at a greater angle to the spool than the first mentioned teeth, companion gears in driving relation with the re-' spective rotary driven elements and with the respective teeth at opposite ends of the spool, the respective series of teeth at opposite ends of the spool and their respective companion gears exerting unequal end thrust pressure in opposite directions, and means to exert axial pressure on the spool in opposition to the greater end thrust of the gears.

6. In a back lash eliminator the combination with a pair of rotary driven elements, a cooperating element engaging both said rotary driven elements, means to drive the rotary driven elements at uniform speed, said means including gears for the respective rotary driven elements and having oppositely and unequally inclined teeth and a spool member meshing at its opposite ends with C the respective gears provided with a means to exert axial pressure on said spool whereby a pre-' determined relative movement of one ofvv said rotary driven elements is efiected.

7. In combination with an hydraulically connism for connecting said power means with the a pair of driving helical gears of opposite and unequal tooth angularity rotating as a unit, hydraulic means to exert axially directed pressure to shift said unit, a pair of independent helical gears meshing respectively with the gears of said unit; a lead screw connected with the support, a pair of nut members cooperating with said lead screw and respectively rotated by said independ-- ent gears whereby upon longitudinal shifting of said unit said independent gears will be given a limited differential rotational adjustment to eliminate lost motion between the nut members and the lead screw, and means to render said hydraulic means ineffective when said power means is adjusted to yield a quick traverse rate.

8. In a machine tool having relatively movable work and tool supports, and a power operable mechanism for effecting relative movement therebetween at diflerent rates, the combination of means for coupling said mechanism for effecting said relative movement without lost motion therebetween, comprising a lead screw connected tothe moving support, a pair of relatively rotatable nut members threaded on the screw and having integral helical gears therewith, intermeshing helical gears of opposite angularity respectively movable as a unit, a fluid operable piston for applying an axial thrust to said helical gear unit to eliminate back lash between said cooperating nut members .and the lead screw, and means operablewhen said power mechanism is adjusted for yieldingbne oi said rates to admit fluid pressure to said piston.

9. In a machine tool having relatively movable work and tool supports and a power operable mechanism for eifecting said relative movement at diiferent rates, the combination of means for coupling said mechanism for eifecting said movement comprising a lead screw and a pair of cooperating nut members, a first pair of intermeshing helical gears for rotating one of said nut members, a second pair oi. intermeshing helical gears of opposite angularity to the first pair for rotating the second nut member, the driving members of the gear pairs being connected together with rotation as a unit, means to impart an axial thrust to said'unit to efiect a'diflerential rotational adjustment of the nut members, said means operating to maintain said adjustment during rotation of the nut members by said power mechanism.

10. In a machine tool having relatively movable work and tool supports and a power operable mechanism for efiecting said relative movement at different rates, the combination of means for coupling said in cm. for effecting said movement comprising a lead screw and a pair of cooperating nut members, a first pair of intermeshing helical gears for rotating one of said nut members, a second pair of intermeshing helical gears of opposite angularity to the first pair for rotating the second nut member, the driving members of the gear pairs being connected together for rotation as a unit, means to impart an axial thrust to said unit to effect a differential rotational adjustment of the nut members, said means operating to maintain said adjustment during rotation of the nut members by said power mechanism, and control devices for rendering said means ineffective during rotation of said nut members at a predetermined one of said rates of said power mechanism.

11. In a machine tool having relatively movable work and tool supports and a power operable mechanism for effecting said relative movement including a feed train and a rapid traverse train, the combination of a final drive mechanism including a lead screw connected to the movable support and a pair ofcooperating nut members, one of said nut members having surfaces thereon which lie in a plane which angularly intersects the axis of the nut members, the other nut member having surfaces thereon but of opposite angularity, means movable parallel to the axis of said nut members and simultaneously engageable with said surfaces for effecting a limited differential rotational adjustment of said nut members to eliminate back lash in their connection with the lead screw, and control devices for rendering said means effective when the feed train is connected for movement of the support and for rendering said means ineffective when said rapid traverse train is connected for movement of the support.

12. In a machine tool having relatively movable work and tool supports, and a reversible power mechanism for effecting relative movement therebetween, the combination of means for coupling said mechanism for efiecting said movement including a lead screw and a pair of cooperating nut members, said nut members hav- ',ing inclined driving surfaces thereon of opposite angularity, driving members for each nut member having cooperating angular surfaces whereby during one direction of rotation of said nut members a resultant thrust in one direction will be produced in said driving members and upon rotation the thrust resultant will be reversed, by-

drauiic means for opposing said resultants, and 

